1. Field of the Invention
The present invention generally relates to confocal imaging systems, and more specifically to tandem scanning in confocal optical microscopy to provide complete images and height measurements, and inspection.
2. General Description of Background
Recently, there has been an increased interest in the use of confocal optical microscopy, especially in the biological fields. In a conventional imaging system, when a part of an object to be imaged is axially displaced from the best focus, the image contrast decreases, but the brightness remains constant so that such displaced unfocused parts of the image interfere with the view of focused parts of the object. However, in a confocal imaging system the brightness of an object decreases rapidly with axial displacement from best focus so that substantially only focused parts of an object are visible.
In a conventional point-confocal microscope, light from a point source is focused within a very small space, known as a spot. The microscope focuses any light reflected or transmitted from the spot onto a point detector. In a reflecting point-confocal microscope the incident light is reflected by any object in the spot. Any light which is reflected by an object outside of the spot is not well focused onto the detector, thus it is spread out so the point detector receives only a small portion of such reflected light. In a transmitting point-confocal microscope, incident light is transmitted unless blocked by any opaque or semi-opaque item in the spot. A small item outside of the spot, whether in front of or behind the spot, will not block a significant amount of the transmitted light. But a larger item, either in front or behind the item in the spot, can block the light, thus preventing determination of the presence of the item in the spot. Generally, the point source and point detector are approximated by placing masks containing a pinhole in front of a conventional light source and conventional detector, respectively.
Similarly, in a conventional slit-confocal microscope system, light from a line source is focused into a very narrow elongate space, which is also known as a spot. The slit-confocal microscope focuses any light reflected or transmitted from the spot onto a line detector. The line source and line detector can be approximated using a mask with a slit in front of a conventional light source and row of conventional detectors, respectively. Alternately, a line source can be approximated by sweeping a focused laser beam across the object to be imaged or inspected.
Since only a small portion of the object is imaged by the confocal microscope, either the object to be imaged must be moved, or the source and detector must be moved, in order to obtain sufficient image data to produce a complete two-dimensional view of the object. Previously, slit-confocal systems have moved the object linearly in a direction perpendicular to the slit to obtain successive lines of image data. On the other hand, point-confocal systems having only one pinhole have to be moved in a two-dimensional manner in order to acquire two-dimensional image data. The raw image data are typically stored and later processed to form a full viewing image of a two-dimensional cross-section of the object that was inspected or imaged. In one previous system known as the Tandem Scanning Microscope, a spiral pattern of illumination and detector pinholes are etched into a Nipkow disk so, as the disk rotates, the entire stationary object is scanned. The Tandem Scanning Microscope allowed stationary objects to be viewed and photographed in real-time.
Application of confocal microscopes to inspection of electronics has been suggested in Zapf et al., Microelectronic Engineering, 5:573 (1986) and Lindow et al, Micron and Submicron Integrated Circuit Metrology, SPIE, vol. 565, p. 81 (1985). The axial discrimination provided by confocal systems may make them useful in the semiconductor manufacturing environment. For example, such systems could provide for improved inspection of height dependent features such as delamination, blisters, and thickness of structures and coatings. However, there are some problems associated with using confocal imaging systems for inspection of electronics. For example, single pinhole systems require too much time for scanning the object in two directions; optical systems for scanning a laser beam over the object are complex; and the spinning disk approach used in the previous Tandem Scanning Microscope resulted in alignment and maintenance problems. Thus, none of these systems can be configured for rapid and/or reliable application, especially in the field of inspection or imaging.
The number of different depth slices required (and therefore the amount of image data collected) depends upon the range of height that must be measured as well as the desired height resolution and performance of the optical system. For typical electronics inspection, images of 10 to 100 different depth slices would be required. Furthermore, data in several color bands may be required to differentiate materials. In previous confocal imaging systems, a separate scan was required for each desired elevation, and if data for multiple color bands was desired, then multiple scans at each elevation were required.